Magnetic-field sensors which incorporate Hall-effect devices are well known in the art. One example of a magnetic-field sensor is disclosed in European Patent application 0 548 391 A1 entitled OFFSET-COMPENSATED HALL SENSOR, by S. Mehrgardt et al. published on Jun. 30, 1993, and assigned to Deutsche ITT Industries the assignee herein. The magnetic-field sensor disclosed in 0 548 391 A1 comprises a Hall-effect device, a power supply, and an evaluating facility which can be supplied with a Hall signal from the Hall-effect device. The evaluating facility disclosed therein includes an input amplifier, a storage element, and a signal superposition unit. A magnetic-field sensor such as the one disclosed in 1445 is frequently implemented as a monolithic integrated circuit comprising the Hall-effect device, the voltage supply, and the evaluating facility. Such a combined circuit is generally fabricated using conventional silicon integrated circuit processing techniques, such as a bipolar or a MOS.
The accuracy of such a magnetic-field sensor can be increased by compensating for the offset signal component of the Hall-effect device by superposition of a first and a second measurement signal. The offset signal component of the Hall-effect device is caused by mechanical stresses in the environment of the Hall-effect device, i.e., in the crystal structure of the monolithic component. Compensation is achieved during the determination of a first measurement signal, wherein terminal pairs of the Hall-effect device, which are connected to the power supply and the evaluating facility, are reversed with respect to the determination of a second measurement signal. Through the reversal of the terminal pairs, a changeover of the power-supply and the evaluation terminals of the Hall-effect device, henceforth called "terminal-pair changeover", is achieved. The geometry of the Hall-effect device and the terminal pairs causes the resulting useful signal components of the measurement signals before and after the terminal-pair changeover to be in phase, whereas the resulting offset signal components of the Hall-effect device are opposite in phase to one another. By adding the measurement signals produced before and after the changeover, which is done in the evaluating facility, the offset signal component of the Hall-effect device is eliminated. To simulate this effect, a Hall plate is thought of, to a first approximation, as a resistance bridge which is balanced in the presence of a magnetic field. The offset signal component results from resistance changes caused in the crystal of the monolithic component by piezoelectric effects and from lithography inaccuracies, etc.
Although the offset signal component of the Hall-effect device is compensated for in such magnetic-field sensors, the accuracy of these sensors is still reduced by the offset signal components of the electronic components in the evaluating facility. For example, when the evaluating facility adds the first and second signals to compensate for the offset signal component of the Hall-effect device, the offset signal component of the input amplifier of the evaluating facility is added.
It is, therefore, a primary object of the present invention to provide an improved magnetic-field sensor which displays substantially greater accuracy when compared with prior art magnetic-field sensors.